Low threshold voltage circuit employing a high threshold voltage output stage

ABSTRACT

An amplifier circuit uses low threshold voltage devices for mid rail response in low voltage applications, but uses a high threshold voltage device at an output stage of the amplifier to generate an output from the amplifier.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/484,556; filed Jul. 2, 2003; and titled “LowThreshold Voltage Circuit Employing A High Threshold Voltage OutputStage.”

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The embodiments of the invention relate to integrated circuits and, moreparticularly, to an output stage of an amplifier.

2. Description of Related Art

In the design of various integrated circuit components, the choice ofselecting a particular characteristic for the component depends on manyfactors. For example, when an amplifier is designed for an integratedcircuit, the components (such as transistors) comprising the amplifierhave properties designed into them based on desired characteristics forthe integrated circuit. One of these electrical properties is thevoltage selected to power the components of the amplifier.

Since today's integrated circuits are designed with power consumption inmind, many transistor circuits are now designed to utilize lower supply(also referred to as rail) voltages. The lower supply voltage allows theintegrated circuit to consume much less power. The signal levels may becorrespondingly smaller as well in relation to the supply voltage. Inmany instances it may be desirable that the transistors drop as littlevoltage as possible to reduce power consumption. Accordingly, in asituation where a lower supply voltage is being utilized, it may bedesirable for the transistors to have low voltage drop across thejunctions. That is, the transistors may be designed to have a lowthreshold voltage (V_(T)) to operate in the lower rail voltageenvironment.

Generally, different V_(T) devices are optimized for different voltageranges. High V_(T) devices may be used in circuits, such as chip I/O(input/output), and tolerate higher voltages. Low V_(T) devices may beused for high-speed lower voltage (and lower power and area)applications in the internal circuitry of the chip. Therefore, lowvoltage devices with low V_(T) are desirable for analog applications,because the supply voltage may be less and less power may be consumed.However, the use of low V_(T) device may make some circuits difficult toimplement.

For example, in an amplifier where the transistors are operating insaturation (where V_(DS)>V_(GS)−V_(T)), there is a minimum value forV_(DS) for the circuit to operate properly. Where multiple transistorsare involved, the V_(GS)−V_(T) value may need to have a higher valueover all process corners. Also, to keep current flowing at the outputstage, the gate-to-source voltage V_(GS) should be above a V_(T).Combining these factors, it is difficult to achieve this over allprocess corners with a thin gate, low V_(T) device. One solution is toimplement an intermediate level shifting stage(s). However, this levelshifting stage at the output is a limiting factor in reducing supplyvoltage.

SUMMARY OF THE INVENTION

An amplifier circuit in which a low threshold voltage stage is used,wherein active load components of the low threshold voltage stage have alow threshold voltage value. The low threshold voltage stage is followedby a high threshold voltage stage. The high threshold voltage stage hasan active component that operates with a high threshold voltage. In oneapplication, the low threshold voltage stage operates at a lower supplyrail voltage, but the following high threshold voltage stage allows forhigher signal swing at the output. In one embodiment, the thresholdvoltage characteristics of the components are obtained by havingdifferent oxide thicknesses.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block schematic diagram illustrating an embodiment of theinvention in which a low threshold voltage circuit or device has a highthreshold voltage output stage.

FIG. 2 is a circuit schematic diagram showing one implementation of thecircuit/device diagram of FIG. 1.

FIG. 3 is a more detailed schematic diagram of the circuit of FIG. 3.

FIG. 4 is an embodiment of a low threshold voltage operational amplifierimplementing a high threshold voltage output stage.

FIG. 5 is a block schematic diagram of an example integrated circuitchip operating as an audio system on a chip.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The embodiments of the present invention may be practiced in a varietyof settings in which low threshold voltage (V_(T)) components areimplemented on an integrated circuit chip. In the following description,references are made to low threshold voltage components or devices andhigh V_(T) components and devices. The actual value of the thresholdvoltage for low V_(T) and high V_(T) devices vary depending on thecomponent, the process utilized to manufacture the component, voltagesapplied to the circuit, etc., as well as other parameters. Generally,V_(T) values will depend on the device, such as a transistor, but arewithin a specified range for a component. Accordingly, the comparison ofthe threshold voltages of a low V_(T) device versus a high V_(T) deviceis a matter of relative difference. Thus, low V_(T) devices have lowerV_(T) values as compared to high V_(T) devices in a particularapplication environment.

Referring to FIG. 1, a block schematic diagram illustrates oneembodiment of the invention. In FIG. 1, block diagram 10 represents adevice or a circuit which resides within an integrated circuit (IC)chip. The device or circuit represented by diagram 10 may be comprisedof one of a variety of circuit components. The device/circuit iscomprised of low V_(T) components 11, such as transistors, includingField Effect Transistors (FETs), which provide a particular operation,such as amplification. An output stage of the device/circuit iscomprised of a high V_(T) output stage 12. An output from the high V_(T)stage 12 is noted as V_(OUT). That is, the device or circuit iscomprised of low V_(T) components 11 which provide a circuit operation,but instead of generating the output utilizing low V_(T) components, theoutput of the low V_(T) components 11 are sent to the high V_(T) outputstage 12 and V_(OUT) is then obtained from the output stage 12.

Referring to FIG. 2, one embodiment for implementing the device/circuitdiagram 10 of FIG. 1 is shown in an amplifier circuit 20. Amplifiercircuit 20 is comprised of a low V_(T) amplification stage 21 and a highV_(T) output transistor 23. In the particular embodiment shown in FIG.2, low V_(T) amplification stage 21 is an operational transconductanceamplifier (OTA), comprised of transistors having low threshold voltages.The input to the amplifier is noted as V_(IN)+ and V_(IN)−. The outputfrom OTA stage 21 at node 22 is noted as voltage V1. Node 22 is coupledto the gate of transistor 23. In the embodiment of FIG. 2, a currentsource 24 represents the current source to the drain of transistor 23.The output V_(OUT) is derived at node 25, which is also at the drain ofthe transistor 23. Accordingly, transistor 23 operates as the outputstage 12 of FIG. 1 and separates the low V_(T) OTA stage 21 from theoutput node 25. Generally, the output voltage V_(OUT) at node 25 iscoupled to a load, such as a subsequent stage operably coupled toreceive V_(OUT.)

In this particular embodiment of FIG. 2, the transistors of the lowthreshold OTA stage 21 obtain low V_(T) characteristics by having thingate oxide thickness. The high V_(T) transistor 23 derives the higherV_(T) characteristic by having thick gate oxide thickness. Again, sincethe low and high designations are relative, it may be better stated thatthe transistors comprising the OTA stage 21 have thinner gate oxidethickness than the gate oxide thickness of transistor 23. The actualoxide thickness difference will depend on a number of factors, includingthe particular manufacturing process used.

In one embodiment using 0.18 micron process technology, an NMOS(N-channel metal oxide semiconductor) transistor has a V_(T) ofapproximately 0.51 volts, while the thick gate NMOS transistor 23 has athreshold voltage of approximately 0.79 volts. These relative thresholddifferentials are obtained in one embodiment by having the gate oxidethickness of approximately 41 angstroms for the low V_(T) transistorsand approximately 68 angstroms oxide thickness for the thick gatetransistor 23. If an embodiment of amplifier circuit 20 is implementedin PMOS (P-channel MOS) technology, the thin gate transistor V_(T) isapproximately 0.51 volts, but the thick gate PMOS transistor V_(T) isapproximately 0.67 volts. Again, it is to be appreciated that thesevalues are provided as example embodiments of implementing the inventionand that actual values will vary with other embodiments.

FIG. 3 is an enlarged and more detailed schematic diagram 30 of theamplifier circuit 20 of FIG. 2. The output stage of FIG. 2 is shown asoutput stage 50 in FIG. 3. P-channel transistors 40, 41 and 42 comprisethe biasing current source and the input differential pair for an OTAstage 31. P-channel transistors 32, 33, 34, 35 and N-channel transistors36, 37, 38, 39 form the active load of the OTA stage 31. The output ofthe OTA stage 31 is at node 45 and is shown as voltage V₁. Voltage V₁ iscoupled to output stage 50 comprised of P-channel transistor 52 andN-channel transistor 51. The output V_(OUT) is obtained at node 53 atthe junction of the drains of transistors 51 and 52. Essentially,transistor 52, in this example, is operating as the current source 24 ofFIG. 2.

With this example embodiment, transistors 40, 41, 42, 32, 33, 34, 35,36, 37, 38 and 39 are low V_(T) transistors. That is, the gate oxidethickness is of such thickness that the V_(T) of these transistors arein the lower range for N-channel and P-channel transistors,respectively. The output transistor 51 is a high V_(T) transistor byvirtue of having its gate oxide of sufficient thickness so that theV_(T) of the transistor 51 is in the higher range for such transistorsor at least higher than the oxide thickness for the transistors of OTAamplifier 31. Transistor 52 may be a high V_(T) device or a low V_(T)device, depending on the particular design or application.

As noted in the Background section, generally different V_(T) devicesare optimized for different voltage ranges. High V_(T) devices may beused in circuits, such as chip I/O (input/output), and tolerate highervoltages. Low V_(T) devices may be used for high-speed lower voltage(and lower power and area) applications in the internal circuitry of thechip. Therefore, low voltage devices with low V_(T) are desirableapplications where the supply voltage may be lower.

With the circuit of FIG. 3, the transistors of stage 31 are in a cascode(or stacked) arrangement. It is typically important for cascodetransistors to have low V_(T), because the stacked transistors of stage31 operate in saturation within the supply rail, V_(DD). However, outputstage 50 may have a high V_(T) device, since it is cascaded.

Referring to FIG. 4, one example implementation of an embodiment of theinvention is shown. In this embodiment, analog amplifier 70 is comprisedof an amplifier stage 71 having low V_(T) components. An output stage 72of the analog amplifier 71 is comprised of a component or componentshaving high V_(T), or the threshold voltage is at least higher than thethreshold voltage of the other components of amplifier 71. An input,noted as V_(INPUT), is coupled to the negative input of the amplifierthrough a resistor 73 (also noted as R₁). An analog ground (V_(AG)) iscoupled to the positive input of the amplifier 71. A feedback of theoutput voltage V_(OUT) is coupled back to the negative input of theamplifier 71 through a feedback resistor 74 (also noted as resistor R₂).Thus, analog amplifier 71, having low V_(T) components but a high V_(T)output stage, operates as a typical operational amplifier. It is to benoted that the various embodiments described above are examples only andthat many other embodiments may be implemented to practice theinvention.

A number of advantages are derived by having a high V_(T) output stageto a low V_(T) circuit. Typically, a low V_(T) device allows the use oflower rail voltages (such as supply voltage V_(DD)). Similarly, highV_(T) devices typically require higher rail voltages due to the higherthreshold voltage being dropped across the transistor. Thus, circuitsemploying low V_(T) devices may operate from lower rail voltages. In oneembodiment the rail voltage may operate at 1.56 volts. In anotherembodiment the rail voltage may be 1.35 volts and still in otherembodiments the rail voltage may be less than 1 volt.

However, having all low V_(T) devices within a circuit may havedrawbacks as was noted above. Biasing may be difficult to maintain nearthe middle of the rail voltage and its return. Mid-rail operation isdesired, since it allows substantially equal headroom to both P-type andN-type transistors. Accordingly, the output voltage of the amplifier(such as the described OTA amplifier) is set by the V_(GS) of theN-channel transistor of the output stage (such as transistor 51 of FIG.3). Attempting to do this adjustment with a low V_(T) device usuallyresults in excessive current and, thus higher power consumption. Thus,low V_(T) devices used alone may require some other adjustment, such asthe use of a level shifter. With the high V_(T) output stage, biasingmay be provided without such level shifting.

When the output stage is maintained as a single transistor, only asingle Vgs (e.g. from transistor 51) sets the direct current (DC)voltage bias on node 45. This reduces the variations that may enter dueto stacked threshold voltages when multiple transistors are involved,such as in the case when level shifters may need to be employed to setthe DC bias. In the embodiments described above, a single transistorhaving a high V_(T) value may be used to set the DC bias near mid-railto improve the signal swing of the output voltage V_(OUT).

As noted above, the difference in the threshold voltage values isobtained by adjusting the thickness of the transistor gate oxide in thelow V_(T) stage and/or the high V_(T) output stage. However, it is to benoted that other techniques may be employed to obtain the differentV_(T) values between the two portions of the circuit. In someembodiments, the same oxide thickness may be employed but otherdifferences, such as material or doping differences, may be employed toobtain different V_(T) values. Thus, the invention is not limited toobtaining different V_(T) values for the transistors strictly byadjusting the thickness of the gate oxide.

It is to be noted that various embodiments of the invention may beemployed in a variety of integrated circuit chips. One exampleembodiment is illustrated in FIG. 5. Referring to FIG. 5, an exampleintegrated circuit (IC) 100 is shown in which one embodiment of theinvention is implemented within IC 100. The example IC 100 is a singleIC chip that implements a complete audio system. It is to be noted thatthe example embodiment of FIG. 5 implements a complete audio system on asingle chip, but other embodiments of the invention may incorporate oneor more integrated circuit chips to provide a complete system or partsof a system.

As illustrated in FIG. 5, a variety of blocks are noted within theconfines of IC 100. The various blocks exemplify hardware components,software and interfaces resident within IC 100. The example audio systemof IC 100 may operate with one or a variety of devices, as illustratedin FIG. 5. Accordingly, a CD (compact disc); LED (Light EmittingDiode)/LCD (Liquid Crystal Display) displays, buttons and/or switches;MMC (Multimedia Card)/SD (Secure Digital) cards; I2C peripherals;SmartMedia, Compact Flash, NOR Flash, NAND Flash, and/or hard drivedevices; and memory, such as SDRAM (Synchronized Dynamic Random AccessMemory) are some components that may be coupled to IC 100 through an I/O(input/output) pin multiplexer 110, as is illustrated in FIG. 5. Thesevarious multiplexed connections are coupled to respective interfaces, asshown in FIG. 5. These interfaces include CD control interface 111; I2Sand CD synchronization interface 112; GPIO (General PurposeInput/Output) interface 113, SPI (Serial Peripheral Interface) interface114; I2C interface 115; Flash/IDE (Integrated Device Electronics)interface 116; and SDRAM (synchronous dynamic random access memory)interface 117.

Furthermore, a USB 2.0 interface 120 allows the coupling of a USBconnection external to the IC 100. In the particular embodiment shown,USB 2.0 interface 120 is compatible with the USB 2.0 and backwardcompatible to a USB 1.1 protocol. A microphone input, radio input and aline input are also available on IC 100 to allow interconnection to amicrophone, radio, or other audio input.

The core of IC 100 is a DSP (Digital Signal Processor) 125, which inthis embodiment is a 24-bit DSP. An on-chip ROM (Read Only Memory) 126and an on-chip RAM (Random Access Memory) 127 operate as memory for DSP125. An analog-to-digital converter (ADC) 130 allows for analog inputsto be converted to digital format for processing by DSP 125. Similarly,a digital-to-analog converter (DAC) 131 is present to convert digitalsignals to analog signals for output in analog form. In this instance,amplified signals through a summation node 132 and headphone amplifier133 generate an amplified analog signal output external to IC 100. Forexample, the analog output may be operably coupled to a set ofheadphones. Also included within IC 100 is a filter and ECC (ErrorCorrection Circuit) engines 140 to provide filtering and errorcorrection operations. Other functions are shown within block 141 toprovide various control and timing functions. These may includeInterrupt Control, Timers, Bit Manipulation Unit, Real Time Clock (RTC),Trace Debug Unit, and error correction just to name a few of theoperations.

Also within IC 100 is a RTC PLL (Real Time Clock/Phase Locked loopcircuit 151, which is coupled to an external crystal 150 to provide anaccurate clocking signal for circuits of IC 100. Memory and peripheralbuses are also present within IC 100 for transfer of data and signals. Atemperature sensor circuit 152 is present to monitor the temperature ofthe IC 100.

In FIG. 5, a rechargeable battery 160 is shown coupled to a lowresolution ADC 161, DC-DC converter 162 and battery charger 163. ADC 161monitors the battery voltage to determine if the battery voltage is suchthat the battery requires charging or if the battery is fully charged.ADC 161 may also monitor the battery voltage to determine if a batteryis present. Thus, if the battery is not present or is removed duringuse, IC 100 detects the absence of the battery through the monitoringprovided by ADC 161. The DC-DC converter 162 converts the batteryvoltage to an operative voltage utilized by components of the IC 100.Battery charger 163 is utilized to charge the battery when an externalvoltage source is coupled to the IC 100.

A variety of batteries may be utilized for battery 160 and, as notedabove, battery 160 is a rechargeable battery. In one particularembodiment, the rechargeable battery is a Nickel Metal Hydride (NiMH)battery. It is to be noted that various other batteries may be utilized,including alkaline cells and lithium ion (LiON) batteries. Generally,battery 160 provides a voltage in the range of 0.9 to 3.6 volts to IC100. In the instance where a NiMH battery is used, the typical range is0.9 to 1.25 volts. Since the voltage from the battery may vary, and/orthe circuitry may require voltages other than what is provided by thebattery, the DC-DC converter 162 provides conversion of the batteryvoltage to one or more voltages utilized on IC 100. In some embodiments,converter 162 may provide more than one DC conversion from the battery.For example, in one embodiment a NiMH battery of 0.9 to 1.25 volts mayprovide nominal chip voltage of 3.3 volts to the load. In another acombination of 3.3 volts and 1.8 volts are provided to the load.

The IC 100 is designed to also operate from other external powersources, when such power sources are coupled to the IC 100. One of thepower sources may be provided through the USB 2.0 interface 120. The USB2.0 protocol specifies the transfer of data by the use of differentialdata lines through a USB link, such as bus 121. The data is generallyprovided on a differential lines (D+ and D− lines). The USB 2.0 protocolalso specifies the presence of a +5 volt DC voltage through bus 121through VBUS and ground (GND) connections. Thus, an external powersource having a voltage of +5 volts may be used as a power source for IC100 through the USB 2.0 interface 120 when bus 121 is coupled to IC 100.In this instance, a USB host provides the 5 volts, while IC 100 operatesas a USB device coupled to the USB host. IC 100 then may use the 5 voltsto power components or circuitry on IC 100 provided the various USBspecification requirements are met. In the particular embodiment of FIG.5, when bus 121 is coupled to IC 100, the 5 volts from the USB hostpowers the internal circuitry, instead of the battery 160. The charger163 uses the 5 volts from the USB host to also charge the battery 160.

In one embodiment, the low-V_(T) amplification/high-V_(T) outputcombination may be utilized in variety of locations. For example, in oneembodiment, amplifiers feeding into the mixer (summation node 132), themixer itself, microphone input amplifier, headphone amplifier 133 andthe multiplexer/amplifier feeding ADC 130 may utilize the low-V_(T)amplification/high-V_(T) output design. It is to be noted that othercircuits may implement the low-V_(T)/high-V_(T) design as well.

1. An apparatus comprising: a low threshold voltage stage powered via afirst power supply voltage, wherein active load components of the lowthreshold voltage stage have a first threshold voltage value; and a highthreshold voltage stage to receive a signal output from the lowthreshold voltage stage and to generate its output, wherein an activecomponent of the high threshold voltage stage has a second thresholdvoltage value which is higher in magnitude than the first thresholdvoltage value, wherein the high threshold voltage stage is powered via asecond power supply voltage, and wherein the second supply voltage isgreater than the first power supply voltage, and wherein the highthreshold voltage stage provides a bias for the low threshold voltagestage.
 2. The apparatus of claim 1, wherein the active load componentsof the low threshold voltage stage are cascoded.
 3. The apparatus ofclaim 2, wherein the active component of the high threshold voltagestage is cascaded to the low threshold voltage stage.
 4. The apparatusof claim 3, wherein the low threshold voltage stage is an operationaltransconductance amplifier.
 5. The apparatus of claim 3, wherein theactive load components of the low threshold voltage stage derive lowerthreshold voltage value by having their gate oxide thickness less thangate oxide thickness of the active component of the high thresholdvoltage stage.
 6. An amplifier comprising: a low threshold voltageamplifier stage, wherein active load components of the low thresholdvoltage stage have a lower threshold voltage characteristic to operateat a lower supply rail voltage; and a high threshold voltage amplifierstage to receive a signal output from the low threshold voltageamplifier stage and to generate its output, wherein an active componentof the high threshold voltage amplifier stage has higher thresholdvoltage characteristic than the components of the low threshold voltageamplifier stage, wherein the high threshold voltage amplifier provides abias for the low threshold voltage amplifier.
 7. The amplifier of claim6, wherein the active load components of the low threshold voltageamplifier stage are cascoded and operate in saturation within the lowersupply rail voltage.
 8. The amplifier of claim 7, wherein the activecomponent of the high threshold voltage amplifier stage is cascaded tothe cascoded low threshold voltage amplifier stage.
 9. The amplifier ofclaim 8, wherein the active component of the high threshold voltageamplifier stage is a single transistor to allow for only a singletransistor gate-to-source voltage to set a direct current (DC) biaspoint at output of the low threshold voltage amplifier stage.
 10. Theamplifier of claim 6, wherein the active load components of the lowthreshold voltage amplifier stage have their gate oxide thickness lessthan gate oxide thickness of the active component of the high thresholdvoltage amplifier stage.
 11. The amplifier of claim 6, wherein the highthreshold voltage amplifier stage operates at a higher supply railvoltage than the low threshold voltage amplifier stage.
 12. Theamplifier of claim 6, wherein the low threshold voltage amplifier stageis an operational transconductance amplifier.
 13. An integrated circuitcomprising: a plurality of audio amplifiers, wherein each of theamplifiers: (a) a low threshold voltage amplifier stage, wherein activeload components of the low threshold voltage stage have a lowerthreshold voltage characteristic to operate at a lower supply railvoltage; and (b) a high threshold voltage amplifier stage to receive asignal output from the low threshold voltage amplifier stage and togenerate its output, wherein, an active component of the high thresholdvoltage amplifier stage has higher threshold voltage characteristic thanthe components of the low threshold voltage amplifier stage, wherein thehigh threshold voltage amplifier stage provides a bias to the lowthreshold voltage amplifier stage.
 14. The integrated circuit of claim13, wherein the active load components of the low threshold voltageamplifier stage are cascoded and operate in saturation within the lowersupply rail voltage.
 15. The integrated circuit of claim 14, wherein theactive component of the high threshold voltage amplifier stage iscascaded to the cascoded low threshold voltage amplifier stage.
 16. Theintegrated circuit of claim 15, wherein the active component of the highthreshold voltage amplifier stage is a single transistor to allow foronly a single transistor gate-to-source voltage to set a direct current(DC) bias point at output of the low threshold voltage amplifier stage.17. The integrated circuit of claim 15, wherein the active loadcomponents of the low threshold voltage amplifier stage have their gateoxide thickness less than gate oxide thickness of the active loadcomponent of the high threshold voltage amplifier stage.
 18. A methodcomprising: utilizing a lower supply rail voltage on components of anamplifier having a low threshold voltage level characteristic to operatethe components in a cascoded arrangement; utilizing a higher supply railvoltage on a last stage of the amplifier to obtain a higher signal swingoutput from the amplifier, the higher supply rail voltage utilized on alast stage component having a higher threshold level characteristic thanthe cascoded components of the amplifier; and providing a bias voltagefrom the last stage of the amplifier to the components of the amplifier.19. The method of claim 18, wherein utilizing the higher supply railvoltage is achieved on a last stage component that has a singletransistor gate-to-source voltage to set a direct current (DC) biaspoint at output of low threshold voltage components of the amplifier.20. The method of claim 18 further including amplifying an audio signalthrough the amplifier.